Custom Link for Image Attachments (not in gallery)

Hey community!

I have been googling myself to death tonight to no avail. I cannot find an answer to something to me seems so simple. Maybe someone here knows the answer!

So, I use a theme called Snapshot Premium which includes the attached image files on the right sidebar of a single post. Love this for several reasons. however, originally when clicking the images they would go to the ugly attachment page.

However, through research I found a piece of code to redirect the attachment page to the current post its attached to. But, I got to thinking since that is possible maybe something is better is.

My question is, how can I get image attachments to link to external URL's? I have seen Custom Gallery Links which is exactly what im looking for minus the "gallery" part. I want this function for stand alone image attachments on posts. Anyone have any ideas? Thanks!